Augmented GPS receivers calculate position measurements by utilizing ranging signals that are received from orbiting ranging satellites and refined by means of auxiliary data broadcast from (1) fixed, earth-based receivers that receive ranging signals from all or some of the same satellites and (2) geostationary satellites that provide data about the ranging satellites. The use of a Differential-GPS (DGPS) system to improve the position calculations of a GPS receiver is well known and need not be described here, except to note that it requires two receivers. In a common DGPS implementation, one receiver is needed to receive ranging signals transmitted by earth-orbiting satellites, and another receiver is needed to receive auxiliary signals that are transmitted by a fixed terrestrial station. The fixed station typically transmits corrections for the position calculations derived from the conventional GPS ranging signals.
In another common DGPS implementation, the auxiliary signals are broadcast from geostationary satellites, and thus a receiver is needed to receive these signals. The geostationary satellites illustratively transmit data about the ranging GPS satellites that can be used to enhance position determinations.
These common DGPS implementations require the use of separate antennas and dedicated demodulators to receive and demodulate the GPS ranging signals and the auxiliary signals, respectively, because of the differences in frequencies and signal formats between the ranging signals and the auxiliary signals.
A typical DGPS implementation consists of the two receivers located in a common housing. Accordingly, when an upgrade is required to the auxiliary receiver to, for example, utilize a different frequency of auxiliary signals; both receivers must often be replaced. The same often holds true when the functionality of an auxiliary receiver is to be added to a device that houses a GPS receiver. When, for example, the GPS receiver is mounted in a handheld device, such an upgrade may require replacing the entire device or at least the ranging receiver to provide sufficient space in the housing to mount both receivers. An additional problem associated with an upgrading that adds or expands the operations of an auxiliary receiver is that the operations of the auxiliary receiver may interfere with the operations of the ranging receiver or the operations of other transmitters or receivers included in the handheld device, such as, for example, blue tooth transmitters and receivers. This is particularly true within the tight constraints of a hand held device.
Antennas that are typically used to receive the auxiliary signal in a DGPS system are electric field (E-field) antennas and magnetic field (H-field) antennas. H-field antennas are generally preferred for systems that are used near significant electrostatic noise sources such as near power lines and where a good ground reference may not be available for an E-field antenna. However, H-field antennas are particularly susceptible to many sources of interference. Co-location of the auxiliary signal receiver of a DGPS system in the same housing with the GPS ranging signal receiver allows the noise from the ranging signal GPS receiver to interfere with auxiliary signal such as those received from an H-field antenna.
Certain previously known DGPS implementations have attempted to solve the interference problem by separating an antenna unit which supports the differential antenna and the GPS ranging antenna from the radio frequency (RF) front end circuitry of the respective receivers. Circuitry is included in the antenna unit which amplifies the GPS signal received from the antenna and superimposes it upon a signal from the differential antenna. The combination signal is sent from the antenna unit via a cable to a receiver system that includes the RF front end circuitry for the GPS ranging signal as well as the RF front end circuitry for the differential signal. The receiver system is thus complex and suffers from many of the same problems discussed above with respect to upgrading or adding functionality, such as use of additional augmentation signals, to the receiver system.